1. Field of the Invention
The present invention relates to a process for preparing a cation-exchangeable membrane which has excellent electrical characteristics and cation selective permeability, chemical resistance, heat resistance and mechanical strength. More particularly, it relates to a process for preparing a cation-exchange membrane which is suitable as a diaphragm for electrolysis of an alkali metal halide.
2. Description of the Prior Art
Cation-exchange membranes have been used in various industries because of cation selective permeability.
For example, the cation-exchange membranes have been used as a diaphragm for electrodialysis by combining it with an anion-exchange membrane or a neutral diaphragm in order to concentrate sea water for producing sodium chloride or to desalt from brine water or sea water for producing fresh water or to recover useful metal salt from a plating waste solution or to treat a drainage in high level or to concentrate and to recover an organic acid or to purify saccharides etc.
They have been also used as a diaphragm for electrode reaction in a fuel battery, or in a dimerization or acrylonitrile to obtain adiponitrile etc.
Various characteristics have been required for using the cation-exchange membrane to these usages. The most important characteristics except special case, are cation selective permeability and durability.
In order to improve the cation selective permeability and the durability, various efforts have been made after finding the ionexchange membranes. As the results, many membranes of made styrene-divinyl benzene type polymers having sulfonic acid groups have been developed as cation-exchange membrane to supply the cation-exchange membranes having satisfactory cation selective permeability and durability.
The condition using the cation-exchange membranes becomes severe as it is found in the treatments of drainages and waste solutions or the electrolysis of sodium chloride. It has been further required to improve the durability of the membranes. The cation selective permeability of the conventional cation-exchange membrane should be further improved.
Although the conventional cation-exchange membranes have excellent blocking property to the permeation of many anions however, the mobility of cations has been remarkably low in an electrolyte solution containing hydroxyl group ions in comparison with the other cases.
The phenomenon is caused because the permeation of hydroxyl group ions can not be effectively prevented as the mobility of hydroxyl group ions in an aqueous solution is remarkably high in comparison with the other anions.
When the cation-exchange membrane is used in the condition containing hydroxyl group ions, for example it is used as a diaphragm for electrolysis of sodium chloride, the current efficiency is lowered disadvantageously because of the phenomenon.
Accordingly, it has been required to develop a cation-exchange membrane having high durability and high resistance to permeation of hydroxyl group ions in these usages of the cation-exchange membrane.
The inventors have studied to develop the cation-exchange membrane for satisfying the requirement of the durability and the permeation of hydroxyl group ions.